What is a block or suite in python? how is indentation related to it?

This chapter is from the book 

Code Blocks and Indentation

One of the most distinctive features of Python is its use of indentation to mark blocks of code. Consider the if-statement from our simple password-checking program:

if pwd == 'apple':
    print('Logging on ...')
else:
    print('Incorrect password.')

print('All done!')

The lines print('Logging on ...') and print('Incorrect password.') are two separate code blocks. These ones happen to be only a single line long, but Python lets you write code blocks consisting of any number of statements.

To indicate a block of code in Python, you must indent each line of the block by the same amount. The two blocks of code in our example if-statement are both indented four spaces, which is a typical amount of indentation for Python.

In most other programming languages, indentation is used only to help make the code look pretty. But in Python, it is required for indicating what block of code a statement belongs to. For instance, the final print('All done!') is not indented, and so is not part of the else-block.

Programmers familiar with other languages often bristle at the thought that indentation matters: Many programmers like the freedom to format their code how they please. However, Python indentation rules are quite simple, and most programmers already use indentation to make their code readable. Python simply takes this idea one step further and gives meaning to the indentation.

If/elif-statements

An if/elif-statement is a generalized if-statement with more than one condition. It is used for making complex decisions. For example, suppose an airline has the following “child” ticket rates: Kids 2 years old or younger fly for free, kids older than 2 but younger than 13 pay a discounted child fare, and anyone 13 years or older pays a regular adult fare. The following program determines how much a passenger should pay:

# airfare.py
age = int(input('How old are you? '))
if age <= 2:
    print(' free')
elif 2 < age < 13:
    print(' child fare)
else:
    print('adult fare')

After Python gets age from the user, it enters the if/elif-statement and checks each condition one after the other in the order they are given. So first it checks if age is less than 2, and if so, it indicates that the flying is free and jumps out of the elif-condition. If age is not less than 2, then it checks the next elif-condition to see if age is between 2 and 13. If so, it prints the appropriate message and jumps out of the if/elif-statement. If neither the if-condition nor the elif-condition is True, then it executes the code in the else-block.

Conditional expressions

Python has one more logical operator that some programmers like (and some don’t!). It’s essentially a shorthand notation for if-statements that can be used directly within expressions. Consider this code:

food = input("What's your favorite food? ")
reply = 'yuck' if food == 'lamb' else 'yum'

The expression on the right-hand side of = in the second line is called a conditional expression, and it evaluates to either 'yuck' or 'yum'. It’s equivalent to the following:

food = input("What's your favorite food? ")
if food == 'lamb':
   reply = 'yuck'
else:
   reply = 'yum'

Conditional expressions are usually shorter than the corresponding if/else-statements, although not quite as flexible or easy to read. In general, you should use them when they make your code simpler.

Indentation is a very important concept of Python because without proper indenting the Python code, you will end up seeing IndentationError and the code will not get compiled.

Indentation

In simple terms indentation refers to adding white space before a statement. But the question arises is it even necessary?
To understand this consider a situation where you are reading a book and all of a sudden all the page numbers from the book went missing. So you don’t know, where to continue reading and you will get confused. This situation is similar with Python. Without indentation, Python does not know which statement to execute next or which statement belongs to which block. This will lead to IndentationError.

In the above example,

• Statement (line 1), if condition (line 2), and statement (last line) belongs to the same block which means that after statement 1, if condition will be executed. and suppose the if condition becomes False then the Python will jump to the last statement for execution.
• The nested if-else belongs to block 2 which means that if nested if becomes False, then Python will execute the statements inside the else condition.
• Statements inside nested if-else belongs to block 3 and only one statement will be executed depending on the if-else condition.

Python indentation is a way of telling a Python interpreter that the group of statements belongs to a particular block of code. A block is a combination of all these statements. Block can be regarded as the grouping of statements for a specific purpose. Most of the programming languages like C, C++, Java use braces { } to define a block of code. Python uses indentation to highlight the blocks of code. Whitespace is used for indentation in Python. All statements with the same distance to the right belong to the same block of code. If a block has to be more deeply nested, it is simply indented further to the right. You can understand it better by looking at the following lines of code.

Example #1:

site = 'gfg'

if site == 'gfg': 

    print('Logging on to geeksforgeeks...') 

else: 

    print('retype the URL.') 

print('All set !') 

Output:

Logging on to geeksforgeeks...
All set !

The lines print(‘Logging on to geeksforgeeks…’) and print(‘retype the URL.’) are two separate code blocks. The two blocks of code in our example if-statement are both indented four spaces. The final print(‘All set!’) is not indented, and so it does not belong to the else-block.

Example #2:

j = 1

while(j<= 5): 

     print(j) 

     j = j + 1

Output:

1
2
3
4
5

To indicate a block of code in Python, you must indent each line of the block by the same whitespace. The two lines of code in the while loop are both indented four spaces. It is required for indicating what block of code a statement belongs to. For example, j=1 and while(j<=5): is not indented, and so it is not within while block. So, Python code structures by indentation.

Note: Python uses 4 spaces as indentation by default. However, the number of spaces is up to you, but a minimum of 1 space has to be used.

What is a block or suite in Python?

What is indentation block in Python?

What do you understand by block and indentation?

Why does indentation matter in Python?